Method of producing a high-strength coil spring

Abstract

 A high-strength coil spring with high fatigue resist­ance comprising a clean steel wire, such as chromium-­vanadium steel wire or chromium-silicon steel wire, formed in the shape of a spring, which is quenched and tempered at lower temperatures to heighten the tensile strength and subjected to a shot peening treatment followed by an elec­trolytic polishing treatment, which does not exert a bad influence on fatigue resistance, to remove surface defects, and a method or producing the same.

Description

[0001] The present invention relates to a high-­strength coil spring and a method of producing the same. The coil spring according to the present inven­tion may be effectively used as a high-strength spring for an engine or as other high-strength springs requiring high fatigue resistance.

[0002] In general, a higher tensile strength is desired for spring materials but it is known that if tensile strength exceeds a certain limit, toughness and fatigue resistance are contrarily reduced.

[0003] In addition, coil springs have been used after forming and then being subjected to a quenching treatment followed by being subjected to a shot peening treatment to add a compressive residual stress to a surface thereof. But an effective shot peening treatment gives a surface roughness Rmax of 6 to 20µm, so that not only has it been impossible to remove surface defects having a surface rough­ness of 6 to 20µm or less,but also impressions due to the shot peening have covered the surface defects to be turned into injured portions and fatigue nuclei in many cases. It goes without saying that the Rmax can be reduced by subse­quent various kinds of polishing treatment but since a surface layer is removed, portions of the outer layer to which a compressive residual stress has been applied with much trouble are lost, whereby the fatigue resistance is reduced on the contrary.

[0004] It is expected that if clean steels, in which the concentration of nonmetallic inclusions has been reduced (such as chromium-vanadium steel and chromium-silicon steel) are used, then the condi­tions for drawing forth the highest fatigue resist­ance as a spring are different from the conventional ones. That is to say, the tensile strength of the present chromium-vanadium steel and chromium-silicon steel is set so that the best fatigue properties may be obtained with a level of inclusions and surface defects in the conventional materials as the base but it can be expected that if merely the problems of surface defects are solved for the clean steels, the fatigue resistance can be improved by still further heightening the tensile strength.

[0005] In view of the above description, the present invention has found a high-strength coil spring with high fatigue resistance using a clean steel wire, such as chromium-vanadium steel wire and chromium-silicon steel wire, by forming it in the shape of a spring, quenching and tempering it at lower temperatures to heighten the tensile strength, and subjecting it to a shot peening treatment followed by subjecting it to an electrolytic polishing treatment, which does not exert a bad influence on fatigue resistance, to remove surface defects and a method of producing the same.

[0006] That is to say, the present invention provides ahigh-strength coil spring, characterized in that its surface roughness Rmax is made 5µm or less by coiling a steel wire formed of steels com­prising C of 0.4 to 1.0 % by weight, Si of 0.1 to 2.0 % by weight, Mn of 0.4 to 1.2 % by weight, Cr of 0.3 % to 1.5 by weight, V of 0.001 to 0.3 % by weight and the remainder of Fe and inevitable impuri­ties, present in the amount of 0.01% or less.

[0007] The invention also provides a method of producing a high-strength coil spring, characterized in that its surface roughness Rmax is made 5µm or less by coiling a steel wire formed of steels comprising C of 0.4 to 1.0 % by weight, Si of 0.1 to 2.0 % by weight, Mn of 0.4 to 1.2 % by weight, Cr of 0.3 to 1.5 % by weight, V of 0.001 to 0.3 % by weight and Fe and inevitable impurities as the rest, of which cleanness is pre­pared at 0.01 % or less, and then subjecting the coiled steel wire to a quenching treatment and a tempering treatment to regulate its tensile strength followed by subjecting to a shot peening treatment and a polishing treatment.

[0008] In the drawings:

Fig. 1(A) to (D) are graphs showing a relation between tempering temperatures and mechanical properties of a chromium-silicon steel wire quenched in oil, in which

Fig. 1(A) shows a relation between temper­ing temperature and hardness;

Fig. 1(B) shows a relation between temper­ing temperature and tensile strength;

Fig. 1(c) shows a relation between temper­ing temperature and reduction of area; and

Fig. 1(D) shows a relation between temper­ing temperature and fatigue strength.

Fig. 2 is a graph showing a distribution of residual stress in the direction of depth of a steel wire after a quenching treatment and a temper­ing treatment by a relation between the distance from a surface and longitudinal residual stress.

Fig. 3(A) and (B) are graphs showing a distri­bution of a residual stress on the inner side of a coil spring in a process (F-1) of the present invention and the conventional process (F-7).

[0009] A steel wire formed of steels comprising C of 0.4 to 1.0 % by weight, Si of 0.1 to 2.0 % by weight, Mn of 0.4 to 1.2 % by weight, Cr of 0.3 to 1.5 % by weight, V of 0.001 to 0.3 % by weight and the remainder of Fe and inevitable impurities is used as a material in the present invention. The reason why the cleanness is prepared at 0.01 % or less is that fatigue fracture due to non-­metallic inclusions contained in the steel wire having the above described chemical composition should be made difficult to be brought about. This can be achieved by devising the deoxidation method such as the optimization of the conditions of a vacuum degassing and a refining slag.

[0010] In addition, the reason why the quenching treatment and the tempering treatment is carried out after the coiling is that if the quenching and tempering treatment is carried out before the coiling, the high-strength material according to the present invention is apt to be insufficient in toughness,and also its sensitivity to a surface defect is strong, so that the proba­bility of breakage during coiling increases.

[0011] Furthermore, the reason why the tensile strength of the chromium-vanadium steel wire quenched in oil for use in the valve-spring by the present invention is increased by 10 % in comparison with the value provided in Table 5 of JIS G-3565,and the tensile strength of the chromium-­silicon steel wire quenched in oil for use in the valve-spring by the present invention is increased by 10 % in comparison with the value provided in Table 6 of JIS G-3566,is that if the surface defects and inclusions are removed, the matrix itself has sufficient toughness and also the fatigue strength can be enhanced even though the strength is enhanced beyondthe conventional value.

[0012] Fig. 1(A) to (D) are graphs showing the influence of lowering the tempering temperature for a chromium-silicon steel wire quenched in oil having a diameter of 4.0 mm, compared with that for the conventional material (tempered at 400°C for obtain­ing the tensile strength corresponding to JIS G-­3566) upon mechanical properties such as hard­ness, tensile strength, reduction in area and fatigue strength.

[0013] It is normal that if the tempering tempera­ture is lowered, as shown in Fig. 1(A), the hard­ness is increased.

[0014] The tensile strength and the fatigue strength (by the rotating bending test) are contrarily reduced, as shown by (b) in Fig. 1(B) and (D). However, in the case where the surface is subjected to the electrolytic polishing, they are contrarily increased up to a certain temperature (250^oC as for the tensile strength and 350^oC as for the fatigue strength) with a reduction of the tempering temper­ature, as shown by (a) in Fig. 1(B) and (D). That is to say, it is found that according to the con­ventional method, the strength of the matrix itself is not sufficiently exhibited due to the surface defects.

[0015] It can be found from the above description that even though the tensile strength after the quenching and the tempering treatment is increased over that of the conventional materials, superior performances can be obtained by reducing the surface defects.

[0016] Fig. 1(C) is a graph showing a comparison of the steel wire (b) as heat treated with the steel wire (a) electrolytic ly polished after heat treatment, regarding the reduction of area.

[0017] The reason why the polishing treatment is carried out after the shot peening treatment is that a zone having the largest compressive residual stress exists at a depth of 100 to 150µm from the surface, as shown by Fig. 2 which is a graph show­ing the distribution of the residual stress in the direction of depth of a steel elementary wire after the quenching treatment and the tempering treatment. Accordingly, it can be thought that if thethickness of a portion to be removed by the polishing treat­ment after the shot peening treatment is l00µm or less, the compressive residual stress of the upper­most surface is rather increased, so that no bad influence is exerted on the fatigue characteristics.

[0018] The steel wire used in the present invention comprises C, Si, Mn, Cr, V, Fe and inevitable impurities,but it is for the following reasons that the content of C is limited within a range of 0.4 to 1.0 % by weight, Si 0.1 to 2.0 % by weight, Mn 0.4 to 1.2 % by weight, Cr 0.3 to 1.5 % by weight and V 0.001 to 0.3 % by weight.

[0019] That is to say, if the content of C is less than 0.4 % by weight, a sufficient strength is not obtained and if the content of C exceeds 1.0 % by weight, shrink cracking is apt to be brought about during the quenching treatment.

[0020] If the content of Si is less than 0.1 % by weight, the heat resistance is deteriorated and if the content of Si exceeds 2.0 % by weight, cracks are apt to be brought about on the surface during the hot rolling.

[0021] If the content of Mn is less than 0.4 % by weight, the quenchability is deteriorated to lead to an insufficient strength and if the content of Mn exceeds 1.2 % by weight, the workability is deteriorated.

[0022] The content of Cr within the range of 0.3 to 1.5 % by weight is effective for the achievement of the superior hardenability and heat resistance.

[0023] The content of V within the range of 0.001 to 0.3 % by weight is preferable in view of the preservation of a superior micronization of crystalline particles and hardenability.

[0024] The present invention will be below described in detail with reference to the preferred embodi­ments.

EXAMPLE 1

[0025] A steel wire with a diameter of 4.0 mm and a chemical composition and purity as shown in Table 1 was produced and springs of the dimensions shown in Table 3 were produced by the manufacturing processes shown in Table 2 from this steel wire. The mechanical proper­ties after the quenching treatment and the temper­ing treatment and the number of cycles to fracture when a fatigue test was carried out at a mean clamping stress τ

of 60 kg/mm² and an amplitude stress τ_a of 45 kg/mm²,are shown in Table 4.

[0026] In addition, the mechanical properties of a sample obtained by coiling followed by being sub­jected to the quenching treatment and the temper­ing treatment in the manufacturing process shown in Table 2 are difficult to measure, so that the mechanical properties of this sample were substi­tuted by characteristic values for a sample obtained by subjecting an elementary wire, which had not been subjected to the coiling, to the same subsequent treatments. In addition, the result of the fatigue test is an average value for n = 4 to 11.

Table 1

Chemical Composition and Purity of Steel Wires to be Tested
	C (wt%)	Si (wt%)	Mn (wt%)	P (wt%)	S (wt%)	Cr (wt%)	V (wt%)	Fe (wt%)	Impurity (%)
A	0.51	0.25	0.78	0.009	0.008	1.02	0.22	Rest	0.003
B	0.46	0.34	0.50	0.008	0.010	1.2	0.25	Rest	0.005
C	0.64	0.13	0.94	0.010	0.005	0.81	0.16	Rest	0.003
D	0.59	0.20	0.48	0.007	0.006	1.10	0.20	Rest	0.042
E	0.58	0.22	0.70	0.006	0.007	0.96	0.23	Rest	0.078

Table 3

Dimensions of Coil Spring
Diameter of elementary wire	4 mm
Average coil diameter	24 mm
Free height	55 mm
Total number of turns	6.5
Effective number of turns	4.5

EXAMPLE 2

[0027] A steel wire with a diameter of 4.0 mm and a chemical composition and a cleanness shown in Table 5 was produced, and springs having the same dimensions as those shown in Table 3 of EXAMPLE 1 were produced by the manufacturing processes shown in Table 6 from this steel wire. The mechani­cal properties after the quenching treatment and the tempering treatment,andthenumber of cycles to fracture when a fatigue test was carried out at a mean clamping stress τ

of 60 kg/mm² and an amplitude stress τa of 50 kg/mm², are shown in Table 7.

[0028] In addition, the mechanical properties of a sample obtained by coiling followed by being subjected to the quenching treatment and the tempering treatment in the manufacturing process shown in Table 6 are difficult to measure, so that the mechanical properties of this sample were substituted by characteristic values for a sample obtained by subjecting an elementary wire, which had not been subjected to the coiling, to the same subsequent treatments. In addition, the result of the fatigue test is an average value for n = 4 to 11.

Table 5

Chemical Compositions and purity of Steel Wires to be Tested
	C (wt%)	Si (wt%)	Mn (wt%)	P (wt%)	S (wt%)	Cr (wt%)	V (wt%)	Fe (wt%)	Impurity (%)
F	0.64	1.43	0.68	0.007	0.013	0.70	0.002	Rest	0.004
G	0.50	1.21	0.52	0.006	0.009	0.54	0.002	Rest	0.003
H	0.77	1.64	0.80	0.010	0.010	1.02	0.003	Rest	0.008
I	0.62	1.47	0.65	0.009	0.015	0.69	0.002	Rest	0.026
J	0.62	1.44	0.68	0.007	0.012	0.68	0.004	Rest	0.089

[0029] It is found from the above described Table 4 of EXAMPLE 1 and Table 7 of EXAMPLE 2 that springs obtained by A-1, A-2, B-1, B-2, B-3, C-1, C-2, F1, F-2, G-1, G-2, G-3, H-1, H-2 and H-3, which are the preferred embodiments of the present inven­tion, are remarkably superior in fatigue useful life time.

[0030] Springs of D, E, I and J types inferior in cleanness, that is D-1, D-2, D-3, D-4, D5, E-1, I-1, I-2, I-3, I-4, I-5 and J-1 are inferior in fatigue resistance. In addition, even in the case where steel wires containing the chemical composi­tions of A and F types are used, springs obtained by the manufacturing processes, in which the elec­trolytic polishing is not or insufficiently carried out, that is springs obtained by the processes of A-3, A-7, F-3 and F-7, are inferior in fatigue resistance.

[0031] Besides, also springs obtained by A-8 and F-8, which are the conventional manufacturing processes of A-7 and F-7 plus the electrolytic polishing process, are inferior to those obtained according to the preferred embodiments of the present inven­tion in fatigue resistance.

[0032] Furthermore, springs obtained by A-4, A-5, A-6, F-4, F-5 and F-6, of which conditions are similar to those in the preferred embodiments of the present invention but the tempering conditions are not suitable, do not exhibit the sufficient fatigue resistance when they are too hard or soft.

[0033] Springs obtained by A-9 and F-9, of which treatment conditions in each process are the same as those in the preferred embodiments of the present invention but the sequence of the processes are different, show problems in that they are inferior in fatigue resistance and difficult to be formed into springs.

[0034] Springs obtained by B-2 and G-2, in which the hot coiling is carried out, and springs obtained by B-3 and G-3, in which the hot coiling is carried out and then the quenching is carried out at that temperature, all exhibit superior fatigue resist­ance if the same low-temperature tempering process and subsequent processes as those in the preferred embodiments of the present invention are adopted.

[0035] It has been found from the above described EXAMPLE 1 and EXAMPLE 2 that a long useful life time of almost 10⁸ as tested by the fatigue test at τ = 60 ± 45 kg/mm² (the fatigue test at τ = 60 ± 50 kg/mm² for chromium-siliconsteelwire) is obtained if a chromium-vanadium steel wire or a chromium-silicon steel wire is subjected to the cold or hot coiling and then quenching and tempering treatment to adjust its tensile strength to be greater than that of a chromium-vanadium steel oil-tempered wire, for use in a valve spring according to JIS G-3565, by about 10 %,or to be greater than the tensile strength of a chromium-silicon steel oil-tempered wire,for use in a valve spring according to JIS G-3566 ,by about 10 %,and the subsequent shot peening followed by the polishing treatment to give the surface roughness Rmax of 5µm or less.

[0036] In addition, graphs showing the distribution of residual stress inside the coil after each process of F-1, which is the preferred embodiment of the present invention, and F-7, which is the conventional example, are shown in Fig. 3. In Fie. 3, a full line shows a longitudinal direc­tion and a dotted line shows a tangential direc­tion.

[0037] It is found from Fig. 3 that in F-1 the residual stress before the shot peening is about ± 0 but in F-7 a residual tensile stress is remained in the longitudinal direction.

[0038] Accordingly, it seems that a compressive residual stress in the longitudinal direction after the shot peening in F-7 is reduced as much as that and the fatigue resistance is deteriorated.

[0039] On the other hand, it is found that in both F-1 and F-7 the compressive residual stress in a zone up to a depth of 20µm from the surface after the shot peening is smaller than that in a zone deeper than 20µm.

[0040] Accordingly, it is found that the removal of the surfaces having the surface roughness of 20µm or less by the polishing treatment has no bad influence upon the fatigue resistances on the whole.

[0041] In F-1 and H-1 in EXAMPLE 2 the thickness of the surface layer removed by the polishing treat­ment was 15µm and that in H-2 was l2µm.

[0042] As above described, the spring obtained by the present invention exhibits remarkably superior fatigue resistance, so that it is very useful for purposes, such as valve springsfor use in car engines requiring reliability.

Claims

1 . A high-strength coil spring, characterized in that it comprises a steel wire com­prising C of 0.4 to 1.0 % by weight, Si of 0.1 to 2.0 % by weight, Mn of 0.4 to 1.2 % by weight, Cr of 0.3 to 1.5 % by weight, V of 0.001 to 0.3 % by weight and Fe and inevitable impurities as the rest and having a cleanness adjusted to 0.01 % or less.

2 . A method of producing a high-strength coil spring, characterized in that a steel wire com­prising C of 0.4 to 1.0 % by weight, Si of 0.1 to 2.0 % by weight, Mn of 0.4 to 1.2 % by weight, Cr of 0.3 to 1.5 % by weight, V of 0.001 to 0.3 % by weight and the remainder of Fe and inevitable impurities present in the amount of 0.01% or less is subjected to coiling to form it into an appointed spring shape, a quenching and tempering treatment to adjust the tensile strength, and a shot peening treatment followed by a polishing treatment to give a surface roughness Rmax of 5µm or less.

3 . A method of producing a high-strength coil spring as set forth in Claim 2, characterized in that the coiling of the steel wire is a cold forming.

4 . A method of producing a high-strength coil spring as set forth in Claim 2, characterized in that the coiling of the steel wire is a hot forming.

5. A method of producing a high-strength coil spring as set forth in Claim 2, characterized in that the coiling of the steel wire is carried out at high temperatures of 820^oC or more and then subjected to the quenching treatment as it is.

6 . A method of producing a high-strength coil spring as set forth in Claim 2, characterized in that the steel wire is heated to 820^oC or more and then subjected to the coil forming at temper­atures of 400 to 600^oC followed by subjecting to the quenching treatment as it is.

Drawing